Hugh: We try and live in the moment as much as possible because the more you think ahead, the more you, you know, you've got two places to go. You've got the hopeful place to go and you've got the despair place to go.

Not looking towards the future is especially tough for Chris and Hugh, considering they’ve spent their careers in forward-thinking Silicon Valley.

I guess we would be considered a kind of power couple although we didn't really look at ourselves that way. He worked on the marketing side and I worked on the PR side. We were constantly busy traveling. It was just hectic.

The couple did make time for each other—eventually getting married and traveling the world together.

A year later, their family doubled. Addison and Cassidy Hempel were born prematurely but the tiny girls were a big hit with their parents.

Hugh: It was paradise. You know, it was hard work. Twins are hard work, so it was all hands on deck. But yeah, they've always been happy babies. Relatively easy. Just went along with the flow.

Chris: They were just little doll babies. They were perfect. They just got into everything, just typical toddlers… opening drawers, getting into makeup, you know just giggling, laughing, just meeting all their major milestones without any problems.

As they grew, Addi and Cassi were delightful, playful, and normal. But shortly after their second birthday, a mysterious illness grabbed a hold of them. It started, simply enough, with a virus: Mononucleosis. Nine months passed, and they couldn’t shake it. Plus, things had gotten worse: now some of their organs were enlarged and doctors were baffled.

Chris Hempel, mother: We did all these tests and the virus was just spinning out of control in their systems, which was unusual, so it kinda threw the doctors off.

Caroline Hastings, an oncologist and hematologist at Children's Hospital in Oakland joined the team of doctors trying to solve the riddle.

Caroline Hastings, oncologist: When I first met the girls I knew they were coming to see me for enlarged spleens and many of the common reasons for enlarged spleens had [already] been ruled out.

Chris: They were just really sick and I don't know, I just—in my gut, I just felt something is really wrong.

Meanwhile, subtle changes were taking place: a stumble here... A tumble there…both girls were losing their balance an awful lot. And even more strange, both Addi and Cassi, who loved to sing and chatter, started slurring their words. After a grueling 18 months of questions, a shocking answer for Chris and Hugh: their twins had an incurable genetic disorder called "Niemann-Pick type C."

Hastings: We don't really have a registry to know how many people are affected in the United States, but likely between 250 and 500 people.

The rare disease prevents Addi and Cassi's cells from processing and eliminating cholesterol. So it builds up, overloading their organs and attacking their brains.

Hastings: They just can't take care of themselves, can't move, they don't eat normally so they're very susceptible to early death.

It was unfathomable to the Hempels. They were told their beautiful little girls were quickly going to become like elderly patients with Alzheimer's or Parkinson's—mentally and physically ravaged.

Chris: You find out your kids have this devastating fatal disease and it just rings in your head again and again and again. You just can't believe it.

Hugh: Both of us were very, you know, emotional. Still are. It's not something you can prepare for.

Chris realized there was no time for self-pity.

Chris: You pick yourself up off the floor and you know I just was—I already was in action mode. I already knew who the doctor was the expert on it, what medication, experimental medication there was for it. I mean everything, all that research had been done.

But just trying to understand the jargon in the science journals became a full-time job.

Chris: It's so complicated and with no medical background, you know, you find yourself reading it ten different times and ten different ways to try to figure out what exactly are they talking about here?

Hastings: It's actually very rare that a parent comes to you and says I've been doing research. She made a point to learn about the biochemistry of this disease, about the genetics of this disease, and really try to understand it and even learn the medical terminology.

Chris's research first led her to change the girls' diet. All meat and high-cholesterol products were eliminated. Then she learned about dozens of drugs and natural supplements to treat Addi and Cassi's various symptoms.

Chris: It's a multi-system type of problem that we're trying to attack so when you have a disease like this, it's gonna take a combination of drugs and different kinds of therapies kinda similar to what they do for HIV/AIDS patients, where it's multiple types of—drugs or supplements.

Despite the hard work, Addi and Cassi were deteriorating in front of their parents' eyes. While they used to amble around and play, now they struggled to keep upright. And while Addi and Cassi used to sing, now they could hardly form a word.

Chris knew her daughters needed a breakthrough—and fast.

Chris Hempel, other: You look back and that's when it really hits you, you know. Then you go, wow, they're deteriorating.

Because of a rare and fatal genetic disorder called Niemann-Pick type C, the girls have stopped developing. And in fact, they're regressing.

Hugh: I find myself once in a while going back and looking at some of the tape and it's hard because they're singing their ABC's and they're running around the house and they're talking in full sentences. It's a reminder of their progression and that's not easy.

In an attempt to stop the downward slide, Chris and Hugh have developed a strict regimen of drugs and supplements.

Chris (showing the drugs): We've had to create our own virtual pharmacy because with a rare disease, there's just nothing in the pipeline that's going to help us. This probably looks, to a lot of people, scary, like what is this mom doing? She's putting all this stuff in her kids? But no, I've really done the research. I've talked to the scientists. I make sure it's not going to be harmful. I look at the amounts.

After being told repeatedly that there was nothing she could do to keep her little girls from dying before their teens, Chris, at last, found a glimmer of hope buried in an obscure scientific article.

Chris: This is the Cyclodextrin…

Not even a drug, Cyclodextrin is an organic sugar compound found in butters and salad dressings on grocery shelves across America. And it appeared to have the ability to break down cholesterol, the very thing that was causing Addi and Cassi's systems to malfunction.

Chris: It tastes kind of like Sweet and Low. I did a lot of research in terms of how much could you put in the sippy cup, in terms of ingesting it, so we started doing that. Then the mice data started coming out showing it doesn't look like it's gonna work if they're eating it.

In order to see results, Cyclodextrin had to be injected directly into the bloodstream. Because Cyclodextrin had never been used in this way on humans, the U.S. Food and Drug Administration had to sign off. The Hempels had to file a lengthy application with the FDA. It was a laborious and intimidating bureaucratic hurdle that would stop most people in their tracks.

Hastings: It's almost unheard of that this would be motivated by a parent or a family.

Next, to back up their application, they had to convince a giant pharmaceutical company to give them the scientific data they collected showing that Cyclodextrin was safe.

Chris: It was pretty disheartening really. They just said: "You know look, we don't provide this information and good luck with getting the FDA to approve it." I kinda hit a real brick wall. The response was not what you would expect when you have two dying children and you're looking for information on a sugar compound.

The tenacious and quick thinking PR exec wouldn't take “no” for an answer. She decided to go over their heads and directly to their parent company.

Chris: It seemed logical to me that here we have Addi and Cassi, two beautiful kids, fighting for their lives. I mean Johnson & Johnson cares so much about kids that surely they would help. As soon as they found out about it, they acted immediately.

Chris got the data, but the FDA still needed more.

Hastings: We were gonna have to develop some type of protocol and monitor the safety of giving this drug and also come up with a way that we were actually going to specifically measure if it made some difference.

After six months of back and forth, Chris won her battle. She got the green light from the FDA to give the girls IV infusions of Cyclodextrin.

On a clear morning in late April, Chris and Hugh prepared their daughters and themselves for a four-day stay at Renown Children's Hospital in downtown Reno. They put on a brave face, but their nerves were palpable. In a few hours, Cyclodextrin was going to flow through an IV drip into Addi and Cassi's veins.

Adam Porath, lead pharmacist: This protocol was different than a typical drug study because for this procedure, we didn't really have a blueprint for what we were supposed to do.

Finally, the two-year quest to find something, anything that could stop the mental and physical decline of Addi and Cassi Hempel begins. Overwhelmed, excited, exhausted, and hopeful, the immensity of this event squeezes out everything else in the room.

Addi and Cassi sit quietly, sharing a bed, staring blankly, as the Cyclodextrin drips into their IV’s. Chris and Hugh Hempel did the impossible—battling the government on behalf of Addi and Cassi—and came out on top.

Now they begin another long wait, to see if their hard won victory pays off in the only way that counts.

Dateline wires the home of a volunteer, Jenny, from top to bottom with hidden cameras. Then she called repairmen to her house for a simple problem we created as a test with Jenny's pool. NBC's Chris Hansen reports.